Adipic acid is a commercial chemical used widely in the production of nylons, especially nylon 6,6, and in the production of polyester polyols. The commercial adipic acid contains various impurities including trash polyvalent metals including iron, copper, vanadium, manganese, cobalt, and nickel as well as nitrogen compounds. Typically commercial adipic acid contains about 0.2-3.0 ppm iron, 0.2-2.5 ppm copper and 1.0-4.5 ppm nitrogen. The impurities in the commercial adipic acid give rise to problems when it is used, for example, fibers spun from the polymer may not dye uniformly, gel may be formed in the continuous polymerizers and yarns that are supposed to be white are sometimes off-white.
To avoid or diminish these problems, a purer form of adipic acid has been produced. This purer form of adipic acid contains less than about 0.8 parts per million (ppm) in total of polyvalent metals and nitrogen. Typically analysis of this purer form of adipic acid will show that it contains about 0.1 ppm iron, less than 0.01 ppm chromium, less than about 0.01 ppm molybdenum, less than about 0.01 ppm nickel, less than about 0.05 ppm cobalt, less than about 0.01 ppm manganese, less than about 0.001 ppm zinc, about 0.02 ppm copper, less than about 0.01 ppm vanadium, and about 0.5-0.6 ppm nitrogen. This purer form of adipic acid has improved the production of nylon polymer and yarn, yielded nylon polymer having reduced gel content, improved run to run consistency, and produced whiter yarns with improved dyeing uniformity; however, when this purer form of adipic acid is used in the formation of certain polyester polyols, color problems arise which did not occur when the commercial grade of adipic acid was employed.
An object of the present invention is an adipic acid composition that can be used to produce better nylon polymer and yarns, and can be used to produce polyester polyols of satisfactory color.